Method for preparing cathodes



United States Patent O US. Cl. 117213 7 Claims ABSTRACT OF THE DISCLOSURE A cathode for an electron tube is prepared by first heating a metal substrate of cathode nickel in an atmosphere consisting essentially of hydrogen and water vapor, the dew point of the atmosphere being in the range of 40 C. to C. Then, a portion of the surface of the substrate is coated with a composition which produces an electron-emissive oxide coating upon subsequent heating at elevated temperatures. The cathode is then ready for assembly into an electron tube.

BACKGROUND OF THE INVENTION This invention relates to a method of preparing cathodes for electron tubes, particularly cathodes of the type in which a metal substrate carries an electron-emissive alkaline earth metal oxide coating. Such cathodes are referred to in the art as oxide-coated cathodes.

Oxide-coated cathodes have been previously described in the literature, for example, in US. Pat. Nos. 2,209,708 to S. Umbreit, 2,631,945 to J. Morrison, Jr. and 3,186,786 to D. MacNair. One type of oxide cathode is comprised of a metal substrate composed of cathode nickel. The substrate may be a substantially pure nickel or, it may be of an alloy which is predominantly of nickel and contains minor amounts of tungsten, silicon, manganese, and magnesium.

The surface of the metal substrate is cleaned and then coated with a combination of heat-decomposable compounds including one or more alkaline earth metal compounds. Usually, this structure is assembled into the electron tube structure and subsequently, as during or after the exhaust and sealing step for making the tube, the substrate and coating are heated to produce the desired electron-emissive oxide coating.

The step of cleaning the metal substrate prior to coating was previously conducted by degreasing the surface of the substrate and then heating the substrate in wet hydrogen gas at about 1100 C. By wet hydrogen is meant a mixture of water vapor and hydrogen gas which has a dew point at about room temperature; that is, about +20 to +30 C. For some structures, dry hydrogen is substituted for the wet hydrogen. By dry hydrogen is meant a mixture of water vapor and hydrogen gas which has a dew point of about 60 C. and lower.

Both wet and dry hydrogen are elfective to clean the surface of the metal substrate. However, it was noted that electron tubes made with substrates that were heated in dry hydrogen (dry-fired) had relatively short operating lives on the average. Careful examination showed that the short lives resulted from the oxide coating peeling from the substrate surface starting at some time during its operating life. Further investigation showed that the same phenomenon occurred in electron tubes made with substrates that were cleaned in wet hydrogen (wet-fired), though to a lesser extent.

SUMMARY OF THE INVENTION According to the invention, the step of cleaning the metal substrate is conducted by heating the metal substrate in an atmosphere of hydrogen and water vapor, which atmosphere has a dew point in the range of 40 C. to 0 C. This atmosphere is referred to as damp hydrogen, and the step is referred to as damp-firing.

For reasons which are not yet understood, and quite surprisingly, the lives of the cathodes and of the electron tubes are extended substantially on the average. This appears to be the result of reduced peeling of the oxide coating from the metal substrate during the operational life of the electron tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS using a cathode nickel alloy composition consisting essentially of nickel containing about 4.0 weight percent tungsten, about 0.025 weight percent magnesium and about 0.035 weight percent silicon. The substrate in this example is in the shape of a cylindrical cup about 0.080 inch outside diameter and about 0.110 inch high. The substrate is degreased, washed in water and then dried. The substrate is then passed through a continuous furnace having a peak temperature of about 1100 C. where it is exposed to temperatures between 1000 C. and 1100 C. for about 7 minutes (and temperatures between 800 C. and 1100 C. for about 10 minutes). The furnace atmos phere consists essentially of water vapor and hydrogen gas and has a dew point of about 25 C. The substrate cools on the way out of the furnace. The cooled substrate is then coated, as by spraying with a triple carbonate composition (containing carbonates of barium, strontium, and calcium and a binder therefor) on the external end portion of the substrate. The coating thickness is about 3.5 mils and the coating Weight is about 0.35 milligram. Upon drying in air, the coated substrate is ready for assembly with other parts into an electron tube.

Example 2 Follow the procedure of Example 1 except substitute dissociated ammonia for hydrogen gas. Such an atmosphere contains about 25 volume percent nitrogen, volume percent hydrogen and a dew point of about 30 C. for the water vapor present.

Example 3 Follow the procedure of Example 1 except substitute a cathode substrate of a cathode nickel composition containing about 99 weight percent nickel, 0.06 weight percent magnesium, and 0.025 weight percent silicon.

Example 4 Follow the procedure of Example 1 except substitute a cathode substrate of a cathode nickel composition containing about 99.2 weight percent nickel, about 0.04 weight percent magnesium, about 0.2 weight percent manganese, and about 0.03 weight percent silicon.

The foregoing examples are illustrative of the invention. There are many variations possible within the scope of the invention. The cathode substrate may be of any size and geometry and still obtain improvements in longer tube life through reduced peeling of the cathode coating. The magnitude of the improvement will, of course, differ with differences in size, geometry and other factors mentioned below. 7

The cathode nickel alloy composition consists essentially of nickel, 0.0 to 10.5 weight percent tungsten, 0.005 to 0.3 weight percent magnesium, and 0.005 to 0.3 weight percent silicon. A preferred cathode nickel composition consists essentially of nickel, 1 to 5 weight percent tungsten, 0.005 to 0.1 weight percent magnesium, and 0.005 to 0.1 weight percent silicon. Magnesium, manganese, silicon and titanium are known to be reducing agents for the oxide coating. The alloy may contain (in weight pera cent) up to about 0.1% Al, 0.04% C, 1.0% Co, 0.20% Cu, 0.1% Fe, 0.20% Mn and 0.008% S as non-essential ingredients. Titanium may be substituted for magnesium. The preferred composition is given in Example 1 wherein the maximum content of nonessential ingredients is preferably (in weight percent): 0.008% Al, 0.02% C, 0.06 Co, 0.10 Cu, 0.10 Fe, 0.05 Mn, and 008% S. The steps of degreasing and washing the substrate may be carried out by any of the techniques known in the art.

The atomsphere for heating the washed substrate is referred to as damp hydrogen. By this is meant an atmosphere which consists essentially of water vapor and hydrogen, wherein the dewpoint for the water vapor is in the range of 40 C. to C. The preferred dew point range is 35 C. to C. Part of the hydrogen may be replaced with one or more neutral gases such as argon, neon, and nitrogen. Example 2, which uses nitrogen in the furnace atmosphere, is illustrative.

The heating step may be conducted in a periodic furnace or a continuous furnace. It is preferred that the atmosphere is continuously flowing through the furnace, as by continuously introducing fresh atmosphere into the furnace and removing a corresponding amount of gas from the furnace chamber.

The heating is conducted for such times and at such temperatures as will produce a surface with a desired character and quantity of nickel oxide. This surface is best produced when the atmosphere is very slightly oxidizing to the substrate surface. This can be achieved in the range of 800 C. to 1200 C. and in a time of several minutes to an hour. At temperatures near 800 C., it is preferred to use higher (wetter) humidities which are near the 0 C. dew point end of the range and longer periods of time. At temperatures near 1200 C., it is preferred to use lower (drier) humidities which are near the 40 C. dew point end of the range and shorter periods of time.

The wetter humidities and higher temperatures are more oxidizing, and the dried humidities and lower temperatures are more reducing with respect to the nickel in the substrate. It is believed that the higher the total content of silicon plus magnesium, the lower will be the preferred humidity of the atmosphere. Also, the greater the content of inert gas in the atmosphere, the lower will be the preferred humidity of the atmosphere.

The cooled substrate may be coated by any convenient coating technique to provide a coating of the desired thickness, weight and texture. In some techniques, as with some spraying methods, the substrate is warmed prior to spraying to aid in producing the desired coating. It is preferred to produce the coating on the substrate within four days after the substrate has been damp fired. Furthermore, no other heating at elevated temperatures should be conducted between the damp firing step and the coating step.

The coating composition may be any composition which upon heating will produce an electron-emissive oxide coating. Such coatings are composed of at least one alkaline earth metal compound which is heat decomposable to an oxide. Combinations of two or more such compounds may be used and may be preferred.

Subsequent to coating the substrate, the substrate is assembled with other parts into a complete electron tube structure including means for heaing the cathode substrate during the operation of the tube. This complete structure is baked at elevated temperatures (typically about 400 C.), the tube exhausted and then sealed. During the baking and exhaust, the substrate is heated up to about 940 C. During this step, the constituents of the coating composition are decomposed, the volatile components are removed, and the remaining oxides are sintered into a firmly adherent coating on the substrate. Also, during this same step, the metal structures of the tube including the cathode substrate are outgassed. Then, the tube is cooled to room temperature and aged with volt ages applied to the electron gun. During aging, the subd strate is heated to about 1100 C. to 1200 C. to activate the cathode coating. The completed tube is then tested.

After testing, the tube is ready for operational use. It has been found that damp-fired cathodes have a substantially longer life than dry-fired cathodes. In one set of tests, tubes containing damp-fired cathodes prepared according to Example 1 except that the atmosphere had a dew point of about 32 C. were compared with similar tubes containing similar dry-fired cathodes which were heated in an atmosphere which had a dew point of about 60 C. The tubes with damp-fired cathodes had a half life (operational to where 50% of samples failed) of about 28 weeks of continuous operation; where: as the tubes with dry-fired cathodes had a half life of about 13.5 weeks of continuous operation.

I claim:

1. A method of making a cathode for an electron tube, said cathode comprising a metal substrate consisting essentially of nickel, magnesium and silicon, said substrate carrying an electron-emissive metal oxide coating thereon, said method including the steps of:

(1) heating the uncoated substrate in an atmosphere consisting essentially of hydrogen and Water vapor, the dew point of said atmosphere being in the range of -40 C. to 0 C.,

(2) and then coating a portion of the surface of said substrate with a composition which produces said oxide coating upon subsequent heating at elevated temperatures.

2. The method defined in claim 1 wherein said uncoated substrate is heated at temperatures between 800 and 1200 C.

3. The method defined in claim 1 wherein said uncoated substrate is heated at temperatures between 1025 and 1100 C. and said atmosphere has a dew point between 35" C. and 20 C.

4. A method of preparing a cathode for an electron tube, said cathode being comprised of a metal substrate composed principally of nickel with 0.0 to 10.5 weight percent tungsten, 0.005 to 0.3 weight percent magnesium, and 0.005 to 0.3 weight percent silicon and a coating of electron-emissive material thereon, said method including the steps of:

(1) heating said substrate in an atmosphere consisting essentially of hydrogen and water vapor, the dew point of said atmosphere being in the range of 40 C. to 0 C.,

(2) and then, without further heating, coating a portion of the surface of said substrate with a composition comprised of at least one alkaline earth compound which is heat-decomposable to an oxide, and a binder therefor.

5. The method defined in claim 4 wherein said substrate is composed principally of nickel with 1.0 to 5.0 weight percent tungsten, 0.005 to 0.01 weight percent magnesium and 0.005 to 0.01 weight percent silicon.

6. The method defined in claim 5 wherein said uncoated substrate is heated at temperatures between 800 and 1200 C.

7. The method defined in claim 5 wherein said uncoated substrate is heated at temperatures between 1025 and 1100" C. and said atmosphere has a dew point between 35 C. and 20 C.

References Cited UNITED STATES PATENTS 3,215,557 11/1965 Kern et al. 117-223 WILLIAM L. JARVIS, Primary Examiner 

